1. Field of the Invention
The present invention relates to a glass ceramic and a temperature compensating member using the glass ceramic, which can be used in many fields, e.g., information communication field, energy related field, electronics field, and the like. In particular, in the optical communication field, they can be used as a part of a device including an optical fiber, e.g., an optical fiber grating, connector or the like, and have a negative coefficient of thermal expansion and can provide a temperature compensation to the device.
2. Description of the Related Art
Recently, an optical fiber is frequently used in the optical communication field and the like. The optical fiber related device is required not to be adversely affected by temperature, on the characteristic of the optical fiber itself.
For example, the connector for the optical fiber is used as an input/output terminal for an optical transmission device or an optical measurement apparatus, or a connector for connecting a plurality of optical cables to each other in an optical transmission path. Such a device having a purpose of fixing, connecting or protecting the optical fibers is required not to be adversely affected on the optical fibers, by the strain caused by expansion or shrinkage of the device according to a temperature change, for example, to have a structure of combination of materials having desired coefficients of thermal expansion, or the like.
Applications of fiber grating are being expanded in wavelength division multiplexing system, for example, as a device for performing dispersion compensation, wavelength stabilization for a semiconductor laser, or the like, which uses a narrow wavelength band selection property thereof. However, it is known that the fiber grating has a temperature dependency of the center wavelength of a reflected light because the effective refractive index of the core portion changes with temperature. Therefore, it is also required that the fiber grating reduces an adverse influence from temperature change as much as possible.
For various types of equipment, apparatus and the like, used not only in the optical fiber related field but also in the energy related field, the information related field, or the like, a material which can adjust the coefficient of thermal expansion of a device or a precision component included in the equipment, apparatus and the like, to an appropriate value, in order to prevent generation of strain or internal stress, caused by a temperature difference, and which is qualified for giving a good dimensional accuracy, a good dimensional stability, a high strength, a good thermal stability or the like, is required.
Conventionally, as materials suitable for various types of devices on the above-described point of temperature change, ceramics, glass ceramics, glass, metal and the like have been used because of having a high heat resistance, a small coefficient value of thermal expansion and the like.
However, these materials have a positive coefficient of thermal expansion, that is, a characteristics of expanding in volume as the temperature increases. Many of other materials used for the devices together with the materials having the positive coefficient have also a positive coefficient of thermal expansion. Therefore, these materials are not necessarily the optimum ones for preventing the adverse influence from the temperature change of the whole device. For this reason, a material having a negative coefficient of thermal expansion, to cancel the positive coefficient of thermal expansion, that is, characteristics of shrinking in volume as the temperature increases, is desired as a material which opposes temperature changes.
As a material having a negative coefficient of thermal expansion, inorganics, e.g., xcex2-eucryptite crystal, Li2Oxe2x80x94Al2O3xe2x80x94SiO2 system ceramics including the xcex2-eucryptite crystal, Li2Oxe2x80x94Al2O3xe2x80x94SiO2 system glass ceramics, ZnOxe2x80x94Al2O3xe2x80x94SiO2 system glass ceramics, lead titanate, hafnium titanate, zirconium tungstate, tantalum tungstate, and the like are known.
U.S. Pat. No. 6,087,280 discloses athermal optical device and a method for producing the device. The device comprises a negative thermal expansion substrate and an optical fiber mounted on the substrate surface and having a grating. In the optical fiber reflective grating device, although the change of the grating center wavelength is approximately 1.9 nm when not attached to the substrate, it comes to be only 0.2 nm when attached to the substrate. The negative thermal expansion substrate comprises a glass-ceramic having xcex2-eucryptite crystal, wherein the substrate has a coefficient of thermal expansion in the range from xe2x88x9220xc3x9710xe2x88x927/xc2x0 C. to xe2x88x92100xc3x9710xe2x88x927/xc2x0 C. in the temperature range of xe2x88x9240xc2x0 C. to 85xc2x0 C. and the hysteresis of the coefficient of thermal expansion is restrained to not more than 20 ppm.
However, the glass-ceramic used in the technique includes many microcracks because of its negative coefficient of thermal expansion and the crystal size to form the microcracks has a diameter larger than 5 xcexcm. There are problems that such a material cannot obtain an enough mechanical strength and impregnates chemical agents or the like easily during a treatment. Impregnation of a chemical agent having a positive coefficient of thermal expansion cancels the inherent negative thermal expansion property of the ceramic. As a result, it is not possible to obtain a desired coefficient of thermal expansion at all.
The crystal phase thereof includes Al2TiO5. It is generally known that because Al2TiO5 has a large anisotropic expansion, the thermal expansion is highly anisotropic for a sintered body and therefore the results of repeated measurements do not coincide with one another in general, and that it is difficult to make the strength large because of the existence of microcracks. There is also another problem that such a material makes the production costs large because a heat-treatment for crystallizing the glass at a temperature of not less than 1,300xc2x0 C. for 3 or more hours is required, in order to obtain an enough negative coefficient of thermal expansion.
U.S. Pat. No. 4,209,229 discloses a glass ceramic comprising a main crystal phase of xcex2-eucryptite or xcex2-quartz solid solution, and the reference says that the material is particularly useful for a protective outer layer for the molten silica or useful for a cladding layer for another optical fiber waveguide member.
However, because the glass ceramic includes much TiO2 to make the crystals fine and therefore lacks for stability of glass, only a thin material thereof can be obtained. Further, there are also problems that a very high temperature, preferably, 1,000xc2x0 C. to 1,300xc2x0 C., is required for crystallizing the glass and that the negative coefficient of thermal expansion of the glass ceramic is not enough, i.e., about xe2x88x922xc3x9710xe2x88x927/xc2x0 C.
U.S. Pat. No. 4,507,392 discloses a transparent glass ceramic material containing xcex2-quartz solid solution as the predominant crystal phase especially suitable for application as a decorative glaze to glass, glass ceramic, and ceramic bodies.
However, because the glass ceramic includes a large quantity of nucleation agents, it is difficult to obtain a material having a large negative coefficient of thermal expansion. The minimum negative coefficient of thermal expansion of the material obtained by the technique of the reference is only xe2x88x9229.4xc3x9710xe2x88x927/xc2x0 C. which is not enough.
Japanese Patent Application Publication (Laid-open) No. Tokukai-sho 63-201034 discloses a method for producing a crystallized glass (glass ceramic) having a negative coefficient of thermal expansion. The method comprises the steps of: mixing volcanic vitreous sediment powder with Al2O3 powder and Li2O powder, heating to melt the mixture, thereafter performing a treatment for removing strain, reheating the performed one at a temperature in a specific range for 12-24 hours, and then slowly cooling it. According to the method of the reference, the maximum negative coefficient of thermal expansion of the material obtained by the technique is about xe2x88x9260xc3x9710xe2x88x927/xc2x0 C.
However, the glass ceramic comprises volcanic vitreous sediments as a raw material and it is not possible to adjust the content of each component, i.e., an alkali metal oxide, an alkaline-earth metal oxide, a transition metal oxide and the like which is necessary to deposit the main crystal phase, other than SiO2 or Li2O. Accordingly, it is not possible to avoid fluctuations of composition thereof and it is difficult to deposit a desired amount of a desired crystal phase. Therefore, the method disclosed in the reference cannot produce a crystallized glass stable in respect of physical properties and quality.
Further, the production method has problems of comprising complex steps, as described in the embodiment of the publication, that is, melting the mixed powder to make cullet, crushing the cullet, and melting it again at a temperature of 1600xc2x0 C., and of higher production costs and unstable productivity because the melting temperature of the glass is very high.
Japanese Patent Application No. Tokugan-hei 11-290029 describes a glass ceramic comprising xcex2-eucryptite solid solution, xcex2-quartz solid solution or the like. The application says that a coefficient of thermal expansion of xe2x88x9225xc3x9710xe2x88x927/xc2x0 C. to xe2x88x92100xc3x9710xe2x88x927/xc2x0 C. can be obtained in a temperature range of xe2x88x9240xc2x0 C. to +160xc2x0 C., and the glass ceramic can be used for various types of temperature compensating members.
However, in fact, the glass ceramic has a large hysteresis of the coefficient of thermal expansion and therefore it is difficult to use it as a temperature compensating member.
Japanese Patent Application Publication (Laid-open) No. Tokukai-hei 2-208256 discloses a ZnOxe2x80x94Al2O3xe2x80x94SiO2 system glass ceramic having a low thermal expansion property, which comprises a main crystal phase of xcex2-quartz solid solution and/or a zinc petalite solid solution. However, the ceramic has the minimum coefficient of thermal expansion having about xe2x88x922.15xc3x9710xe2x88x926/xc2x0 C. (xe2x88x9221.5xc3x9710xe2x88x927/xc2x0 C.) which is not enough.
Because the ceramic contains a large quantity of ZnO component which is apt to sublimate at a high temperature, it is not preferable that a raw material is molten for a long time during formation of parent glass (original glass), as described in this reference. The melting time disclosed in the embodiment is extremely short, i.e., only ten minutes. According to such a short time for melting the material, the components of SiO2 and Al2O3 are not molten sufficiently. As a result, a part thereof is leave to be unmolten even if the melting temperature is high. Accordingly, it is not possible to obtain a parent glass with even quality. As a result, it is not possible to obtain a uniform ceramic even if crystallizing such a parent glass with uneven quality.
If the glass is molten for a long time, e.g., in general, for several hours, the problem of leaving a part thereof unmolten can be solved. However, in this case, because the composition of the parent glass is changed by sublimation of ZnO component, it is also not possible to obtain a uniform ceramic.
Because the melting temperature in the above-described embodiment is very high, i.e., 1620xc2x0 C., higher production costs are required.
U.S. Pat. No. 5,694,503 discloses a package comprising an optical fiber with a refractive index grating, and a support member having a negative coefficient of thermal expansion, to which the optical fiber is attached. As a negative thermal expansion material, a Zr-tungstate-based composition or an Hf-tungstate-based composition is used. In the method, a material having a negative coefficient of thermal expansion of xe2x88x929.4xc3x9710xe2x88x926/xc2x0 C. is obtained by using ZrW2O8 having a coefficient of thermal expansion of xe2x88x924.7xc3x9710xe2x88x926/xc2x0 C. to xe2x88x929.4xc3x9710xe2x88x926/xc2x0 C. The reference says that it is possible to reduce the wavelength shift caused by temperature change by attaching the optical fiber on the support member made of the negative thermal expansion material, with applying an appropriate stress to the grating.
In the material of ZrW2O8 or HfW2O8, complicated steps, that is, adding an appropriate amount of material powder having a positive coefficient of thermal expansion, e.g. , Al2O3, SiO2, ZrO2, MgO, CaO, or Y2O3, to the negative thermal expansion material, to adjust the coefficient of thermal expansion to a desired value, and sintering the mixture to make a sintered body, are required. Therefore, such a material is not suited for mass production. Because different materials must be mixed, suitable technology and facilities are required. The materials do not necessarily have uniform quality. Further, because in ZrW2O8 or HfW2O8, a phase transition occurs at a temperature around 157xc2x0 C. and a flexuous portion is seen in a thermal expansion curve, it is not thermally stable in a wide temperature range.
Each of the publication WO97/14983 and Japanese Patent Application Publication (Laid-open) No. Tokukai-hei 10-90555 discloses a liquid crystal polymer, as a material having a negative coefficient of thermal expansion.
However, because the liquid crystal polymer is a crystalline resin and has a strong crystal orientation characteristic, for example, in an injection molded product, there is a problem of curvature and the like. There is another problem that the one having a large negative coefficient of thermal expansion, e.g., xe2x88x92100xc3x9710xe2x88x927/xc2x0 C., in the direction of orientation, has a large positive coefficient of thermal expansion in directions other than the orientation direction. Because the value of the material property, e.g., bending strength, elastic modulus or the like, also differs depending on the direction, it is difficult to use such a material for a device.
As described above, in the actual circumstances, conventional materials having a negative coefficient of thermal expansion are used little in various fields, e.g., optical communication field, energy related field, information related field, and the like because of having the above problems.
The present invention was developed in view of the problems in the actual circumstances. An object of the invention is to provide a glass ceramic which has a sufficiently large absolute value of negative coefficient of thermal expansion in a temperature range of xe2x88x9240xc2x0 C. to +160xc2x0 C. which is a general temperature range in use in optical communication field, energy related field, information related field, and the like, and which can be produced at a low cost and stably on the points of composition and property, and to provide a temperature compensating member using the glass ceramic.
The inventor has carried on various examinations and researches to attain the above-described object. As a result, the inventor has succeeded in obtaining glass ceramics which improve stability of material, restrain generation of microcracks and have no anisotropic expansion, by heat-treating Li2Oxe2x80x94Al2O3xe2x80x94SiO2xe2x80x94TiO2 system glasses within the range of the specific compositions to deposit fine crystal grains, and has found that the obtained glass ceramics are preferable as temperature compensating material. The present invention has been achieved on the basis of the facts.
That is, in accordance with an aspect of the present invention, a glass ceramic comprises: a main crystal phase which comprises at least one selected from a group consisting of xcex2-eucryptite (xcex2-Li2O.Al2O3.2SiO2), xcex2-eucryptite solid solution (xcex2-Li2O.Al2O3.2SiO2 solid solution), xcex2-quartz (xcex2-SiO2), and xcex2-quartz solid solution (xcex2-SiO2 solid solution),
wherein an average grain size of the main crystal phase is less than 5 xcexcm, and a coefficient of thermal expansion thereof is xe2x88x9230xc3x9710xe2x88x927 to xe2x88x9290xc3x9710xe2x88x927/xc2x0 C. in a temperature range of xe2x88x9240xc2x0 C. to +160xc2x0 C., and a hysteresis of the coefficient of thermal expansion is not more than 20 ppm (xc3x9710xe2x88x926),
The main crystal phase may contain no Al2TiO5 crystal.
The glass ceramic may be substantially free of PbO, Na2O and K2O.
The glass ceramic may comprise the following components:
The glass ceramic may be produced by a method comprising the steps of: melting an original glass, forming the molten original glass and slowly cooling the formed glass, carrying out a first heat treatment for the cooled product at a temperature of 550 to 800xc2x0 C. for 0.5 to 50 hours; and carrying out a second heat treatment for the heat-treated product at a temperature of 700 to 950xc2x0 C. for 0.5 to 30 hours.
The glass ceramic may be produced by a method comprising the steps of: cutting and polishing the glass ceramic, and thereafter carrying out a heat treatment for the cut and polished glass ceramic at a temperature of 200 to 400xc2x0 C. for 1 to 10 hours.
A temperature compensating member comprises the glass ceramic comprising: a main crystal phase which comprises at least one selected from a group consisting of xcex2-eucryptite (xcex2-Li2O.Al2O3.2SiO2), xcex2-eucryptite solid solution (xcex2-Li2O.Al2O3.2SiO2 solid solution), xcex2-quartz (xcex2-SiO2), and xcex2-quartz solid solution (xcex2-SiO2 solid solution), wherein an average grain size of the main crystal phase is less than 5 xcexcm, and a coefficient of thermal expansion thereof is xe2x88x9230xc3x9710xe2x88x927 to xe2x88x9290xc3x9710xe2x88x927/xc2x0 C. in a temperature range of xe2x88x9240xc2x0 C. to +160xc2x0 C., and a hysteresis of the coefficient of thermal expansion is not more than 20 ppm.